This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-104915, filed Apr. 3, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a so-called cone beam X-ray computerized tomographic apparatus which scans an object to be examined with a pyramidal X-ray beam to obtain 3-D information.
2. Description of the Related Art
In a cone beam scan scheme, an object to be examined is scanned with an X-ray beam emitted from an X-ray tube and trimmed into a pyramidal shape by an X-ray stop. The X-ray beam transmitted through the object is detected by a 2-D array type detector. As an X-ray detector of this type, a detector having an array of a relatively small number of line detectors, typically four line detectors, has become widespread. Recent years, however, have witnessed the advent of an X-ray detector having 32 or more arrays of line detectors by using solid-state detecting elements constituted by combinations of scintillator elements and photodiode elements or solid-state detecting elements made of selenium or the like which directly convert X-rays into electric charges. The 2-D array type detector has the form of the cylinder or the plane.
As a cone beam image reconstruction method, the FeldKamp method is generally used. The FeldKamp method is an approximate reconstruction method based on the fan beam convolution/back projection method. Convolution processing is performed by regarding data as a fan projection data on the premise that the cone angle is relatively small. However, back projection processing is performed along an actual ray.
That is, an image is reconstructed by the following procedure:
(1) assigning Z-axis-dependent weights to projection data;
(2) performing convolution for the data in (1) by using the same reconstruction function as that for a fan beam reconstruction; and
(3) performing back projection with respect to the data in (2) along an actual oblique ray having a cone angle.
In such an image reconstruction method, however, the effective height of a field of view changes depending on the radius of the field of view. This problem will be described in detail below.
FIG. 1 is a side view of a field of view whose radius is set to a relatively long length RLL. FIG. 2 is a side view of a field of view whose radius is set to a relatively short length RSS. A radius R of the field of view is set to a length within which a region to be examined, e.g., the head, lungs, body. Note that the xe2x80x9ceffective heightxe2x80x9d of the field of view is defined by the length of the field of view in the slice direction in which the set radius R is maintained (the length of the field of view in the body axis direction of the object).
In the FeldKamp method, since data projected over one rotation are required, the maximum range in which image reconstruction can be done is limited to a cylindrical shape. In this range, the effective height of the field of view within which the radius R is maintained is limited to WLL when the radius is RLL, as shown in FIG. 3A. When the radius is RSS, the effective height is limited to WSS, as shown in FIG. 3B. In this manner, the effective height of the field of view changes corresponding to the radius to which the field of view is set.
It is an object of the present invention to reduce the dependence of an effective height on the radius of a field view in a cone beam type X-ray computerized tomographic apparatus.
According to the first aspect of the present invention, there is provided an X-ray computerized tomographic apparatus comprising an X-ray tube device configured to irradiate an object to be examined with a pyramidal X-ray beam, a detector which has a plurality of detecting elements arrayed in a slice direction in which X-rays transmitted through the object are detected, a data extending unit configured to create virtual data corresponding to an extension region located outside a region in which the detecting elements are arranged in the slice direction on the basis of real data detected by the detecting element, and a reconstructing unit configured to reconstruct image data on the basis of the real data and virtual data.
According to the second aspect of the present invention, there is provided An X-ray computerized tomographic apparatus comprising an X-ray tube device configured to irradiate an object to be examined with a pyramidal X-ray beam, a detector which has a plurality of detecting elements arrayed in a slice direction in which X-rays transmitted through the object are detected, an input device which inputs a radius of a field of view, and a reconstructing unit configured to reconstruct image data about a field of view in which the input radius is maintained within a predetermined length range in the slice direction on the basis of real data detected by the detecting element and virtual data created from the real data.
According to the third aspect of the present invention, there is provided an X-ray computerized tomographic apparatus comprising an X-ray tube device configured to irradiate an object to be examined with a pyramidal X-ray beam, a detector which has a plurality of detecting elements arrayed in a slice direction in which X-rays transmitted through the object are detected, and a reconstructing unit configured to reconstruct image data about a field of view having an arbitrary radius and fixed axis length on the basis of real data detected by the detecting element and virtual data created from the real data. Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.